domingo, 30 de septiembre de 2012

Will Graphene Unlock Secrets of the Big Bang?

Graphene has caused a lot of excitement among scientists since the extremely strong and thin carbon honeycomb-shaped material, just one atom thick, was discovered in 2004. In 2011, NASA'sSpitzer Space Telescope spotted the signature of flat carbon flakes, called graphene, in space --the first-ever cosmic detection of the material -- which is arranged like chicken wire in flat sheets that are one atom thick.

The team of astronomers using Spitzer identified signs of the graphene in two small galaxies outside of our own, called the Magellanic Clouds, specifically in the material shed by dying stars, called planetary nebulae. The N 70 nebula shown above is a "Super Bubble" in the Large Magellanic Cloud (LMC image below), a satellite galaxy to the Milky Way system, located in the southern sky at a distance of about 160,000 light-years.

The infrared-sensing telescope also spotted a related molecule, called C70, in the same region – marking the first detection of this chemical outside our galaxy. According to the astronomers, the graphene and C70 might be forming when shock waves generated by dying stars break apart hydrogen-containing carbon grains.

Physicist Peter Horava, at the University of California, Berkeley, thinks graphene can help us understand what happened immediately after the big bang as well as what's going on near the event horizon of black holes, where the gravitational fields are massive.

Letizia Stanghellini and Richard Shaw, members of the team at the National Optical Astronomy Observatory in Tucson, Arizona, describe how collisional shocks powered by the winds from old stars in planetary nebulae could be responsible for the formation of fullerenes (C60 and C70) and graphene (planar C24). The team was led by Domingo Anibal Garcia-Hernandez of the Instituto de Astrofisica de Canarias in Spain and includes international astronomers and biochemists.

Planetary nebulae originate from stars similar to our Sun that have reached the end of their lives and are shedding shells of gas into space. In this case, the planetary nebulae are located in the Magellanic Clouds, two satellite galaxies to our own Milky Way, that are best seen from the Southern Hemisphere. At the distance of the Magellanic Clouds, planetary nebulae appear as small fuzzy blobs.

However, unlike planetaries in our own Milky Way Galaxy whose distances are very uncertain, the distance to planetaries in the Magellanic Clouds can be determined to better than 5%. With such accurate distances, the research team determined the true luminosity of the stars and confirmed that the objects are indeed planetary nebulae and not some other object in the astrophysical zoo.

Fullerenes, or Buckyballs, are known from laboratory work on Earth and have many interesting and important properties. Fullerenes consist of carbon atoms arranged in a three dimensional sphere similar to the geodesic domes popularized by Buckminster Fuller.

The C70 fullerene can be compared with a rugby ball, while C60 is compared to a soccer ball. Both of these molecules have been detected in the sample. Graphene (planar C24) is a flat sheet of carbon atoms, one atom thick, that has extraordinary strength, conductivity, elasticity and thinness.

Cited as the thinnest substance known, graphene was first synthesized in the lab in 2004 by Geim and Novoselov for which they received the 2010 Nobel Prize in physics. "If confirmed with laboratory spectroscopy - something that is almost impossible with the present techniques - this would be the first detection of graphene in space," said team member Garcia-Hernandez.

The team has proposed that fullerenes and graphene are formed from the shock-induced (i.e., grain-grain collisions) destruction of hydrogenated amorphous carbon grains (HACs). Such collisions are expected in the stellar winds emanating from planetary nebulae, and this team sees evidence for strong stellar winds in the ultraviolet spectra of these stars.

"What is particularly surprising is that the existence of these molecules does not depend on the stellar temperature, but on the strength of the wind shocks," says Stanghellini.

The Small Magellanic Cloud is particularly poor in metals (any element besides hydrogen and helium, in astronomers' parlance), but this sort of environment favors the evolution of carbon-rich planetary nebulae, which turns out to be a favorable place for complex carbon molecules.

The challenge has been to extract the evidence for graphene (planar C24) from Spitzer data. "The Spitzer Space Telescope has been amazingly important for studying complex organic molecules in stellar environments," says Stanghellini.

"We are now at the stage of not only detecting fullerenes and other molecules, but starting to understand how they form and evolve in stars." Shaw adds, "We are planning ground-based follow up through the NOAO system of telescopes. We hope to find other molecules in planetary nebulae where fullerene has been detected to test some physical processes that might help us understand the biochemistry of life."

Meanwhile Horava, at the University of California, Berkeley, has developed a new theory of quantum gravity that reflects the need understand what happened immediately after the big bang or what's going on near the event horizon of black holes, where the gravitational fields are massive.

In the physics of condensed matter, specifically in graphene, a carbon atom one atom thick, whose electrons ping around the surface like balls in a pinball machine and can be described using quantum mechanics. Because the graphene atoms are moving at only a fraction of the speed of light there is no need to take relativity into account.

But cool graphene down to near absolute zero and something extraordinary happens: the electrons speed up dramatically. Now relativistic theories are needed to describe them correctly. It was this change that sparked Horava's imagination. What struck Horava about graphene is that Lorentz symmetry isn't always apparent in it.

Could the same thing be true of our universe, he wondered. What we see around us today is a cool cosmos, where space and time appear linked by Lorentz symmetry - a fact that experiments have established to astounding precision. But things were very different in the earliest moments. What if the symmetry that is apparent today is not fundamental to nature, but something that emerged as the universe cooled from the big bang fireball, just as it emerges in graphene when it is cooled?

Horava tweaked Einstein's equations in a way that removed Lorentz symmetry: a property that keeps the speed of light constant for all observers, no matter how fast they move, time slows and distances contract to exactly the same degree. This led Horava to a set of equations that describe gravity in the same quantum framework as the other fundamental forces of nature: gravity emerges as the attractive force due to quantum particles called gravitons, in much the same way that the electromagnetic force is carried by photons. He also amended general relativity to include a preferred direction for time, from the past to the future -the way the universe as we observe it appears to evolve.

"All of a sudden, you have new ingredients for modifying the behaviour of gravity at very short distances," Horava said in an interview with New Scientist.

By breaking asunder the symmetry between space and time, Horava's theory alters the physics of black holes - especially microscopic black holes, which may form at the very highest energies, which means for the formation of these black holes, and whether they are what they seem to be in general relativity "is a very big question."

Horava gravity might also help solve one of the great unsolved mysteries of modern cosmology: the puzzle of dark matter if the equations of motion derived from general relativity are slightly off this could explain the observed speeds of the stars and galaxies without dark matter playing a role.

"It is possible that some fraction of the dark matter picture of the universe could be coming from corrections to Einstein's equations," Horava said.

Ditto for dark energy: theories of particle physics predict the strength of dark energy to be about 120 orders of magnitude larger than what is observed, and general relativity cannot explain this enormous discrepancy. But Horava's theory contains a parameter that can be fine-tuned so that the vacuum energy predicted by particle physics is reduced to the small positive value that is in line with the observed motions of stars and galaxies.

The ultimate answers will come with Improved observations of supermassive black holes, which contain regions of intense gravity, which could reveal the necessary corrections to general relativity and prove Horava's theory of quantum gravity, in much the same way that unexplained measurements of Mercury's orbit showed that Newton's laws were incomplete, opening the door for Einstein.